It is known to employ lignosulfonate compounds, including sodium salts of lignosulfonates, as an additive, such as a dispersant, in textile dyestuffs and printing pigments. Such lignosulfonate compounds are generally produced as by-product of the wood pulping industry by either the sulfite or kraft process. Such sulfonated lignin products provide three basic functions in dyestuff compositions:
(1) They assist in reducing the dye particles to a fine size; PA0 (2) They maintain a dispersing medium for the dyestuff; and PA0 (3) They are used as a diluent.
The advantages of employing sulfonated lignins as dispersants in dyestuff compositions are based on their unique physical properties which include good compatibility with many dye systems, outstanding disperant characteristics at ambient and elevated temperatures, and availability. There are certain disadvantages in employing lignins, whether they are sulfite lignins or sulfonated kraft lignins, as dispersants. Negative factors in the use of such lignins as dyestuff additives relate to problems of high inorganic salt content, i.e., electrolyte content, when lowered in pH, foaming, high pH, fiber staining, poor heat stability, and high viscosity. These adverse properties are troublesome to dyers and many attempts have been made to overcome these and other disadvantages.
Inorganic electrolyte content of lignin dispersants and dyestuff additives greatly affect their use in a specific dyestuff additive formulation. High electrolyte content of a lignin dispersant imposes unwanted side effects on hydrophobic dyestuffs. In vat dyes, high salt content of the lignin additives can cause harmful rheological effects during storage of the dyes. The viscosity of the oxidized form in the presence of salts generally increases to a level where the dye mixture can only be removed from a storage container with considerable difficulty. Recent use in this country of double strength dyes over powder dyes has necessitated a reduced application level of the lignin dispersants in order to accomodate the increased amount of dye, thus dictating that the dispersant be in its purest state possible.
A number of technological developments have resulted in new methods and processes to modify sulfonated lignins to reduce the negative aspects of employing such materials as dye dispersants without simultaneously causing any major adverse effects upon those properties which render sulfonated lignins desirable as dyestuff dispersants. The following U.S. patents are directed to reacting and modifying lignins to make them more suitable as dye dispersants: U.S. Pat. Nos. 4,001,202 to P. Dilling et al., 4,184,845 to S. Y. Lin; 4,131,564 to P. Dilling; 3,156,520 to L. A. Baisdell; 3,094,515 to K. F. Keirstead et al; 3,726,850 to Detroit; 2,680,113 to E. Adler et al; 3,769,272 to Hintz; 3,841,887 to Falkehag et al; 4,355,996 to P. Dilling et al; and 4,308,203 to Lin.
U.S. Pat. Nos. 2,525,433; 2,690,973; and 3,503,762 disclose the use of lignins as an additive in printing inks, pastes, and gels. The foregoing patent art is cited to show the state of the art and is not intended to be all inclusive of lignin modifications for use as dye additives.
More recently in my copending commonly assigned U.S. patent applications Ser. No. 601,047 and Ser. No. 601,049 filed Apr. 16, 1984, now issued respectively as U.S. Pat. Nos. 4,521,336 and 4,551,151, there is described a process for preparing sulfonated lignins particularly suited for use as dyestuff dispersants, and the products produced thereby, wherein the viscosity of sulfonated lignins can be reduced with improvement in heat stability properties by removing the low molecular weight component of the lignin and thereafter sulfonating the remaining lignin material with sodium sulfite (Na.sub.2 SO.sub.3) and aldehyde at a pH of about 8.0 to 9.2. Also, in my commonly assigned copending U.S. patent application Ser. No. 532,677, filed Sept. 16, 1983, now abandoned, there is disclosed a process for producing sulfonated lignins for use as dye dispersants and additives, and the products produced thereby, wherein dispersability and heat stability of the lignins are improved by post-sulfonation cross-linking of the sulfonated lignins at controlled lower pH levels, typically a pH of between about 7.0 to 8.5.
In the paper industry, lignin is obtained as a by-product from spent pulping liquors, known as black liquor, where lignocellulosic materials, such as wood, straw, cornstalks, bagasse and the like are processed to separate the cellulosic pulp from the lignin. The lignins employed in the process of this invention may readily be obtained from the kraft wood pulping process wherein the natural lignin is present as a sodium salt. In kraft pulping, the wood is subjected to the effects of strong alkali. The lignin forms a soluble sodium salt in the alkaline region which is separated from the cellulose and dissolves in the pulping liquor. The lignin is then recovered from the black liquor by acidification.
Acidification of black liquor containing the lignin salt generally may be accomplished by the introduction of carbon dioxide which converts the phenolic hydroxide groups on the lignin molecule, which are in ionized form, into their free phenolic or acidic form. This conversion renders the lignin insoluble in the black liquor and, as a result, it precipitates out. To precipitate the alkali lignins from the black liquor as water-insoluble products, black liquor initially having a pH of around 13.0 is acidified to about a pH of 10.5 at which point the lignin begins to precipitate. The lignin precipitate can be further purified by reducing the pH level to pH 2, where the lignin is coagulated and washed with water to obtain a lignin product identified as "A" lignin.
Lignin obtained from the kraft, soda or other alkaline processes is not recovered as a sulfonated product, but is sulfonated by reacting the material with a bisulfite or sulfite compound. A sulfonated lignin is understood to be any lignin containing at least an effective amount of sulfonate groups to give water solubility in moderately acid and higher pH solutions.
One of the conventional processes for sulfonating lignin involves sulfomethylation of the alkali lignin by reacting the lignin with sodium sulfite and formaldehyde. Such a process is described in Adler et al U.S. Pat. No. 2,680,113. Sulfomethylation acts upon the aromatic phenolic nuclei of the lignin molecule in such a manner that --CH.sub.2 SO.sub.3 H groups are bonded to the aromatic phenolic ring. It is also possible to sulfonate the lignin side-chain of the aromatic nucleus by sodium sulfite treatment of the lignin in the absence of formaldehyde. Sulfomethylation of the alkali lignin has been carried out generally at a pH level of 9.0 or higher, in order to ensure optimum phenol ionization and solubility of the lignin for sulfomethylation.
Although prior art sodium salts of sulfomethylated lignins existed as relatively pure products when sold as a high alkaline pH composition, their use as a dyestuff additive requires adjustment by a formulator and/or a dyer to an acidic pH level commonly employed in most textile dyeing by the addition of acid. Since the amount of acid required to either neutralize or acidify the lignin corresponds directly to the amount of electrolytes which develop in a given dyestuff formulation, acidification produces increased undesirable electrolyte content in the composition. In lowering the pH of the lignosulfonates, the phenolic groups which convert from ionized form to acid form initially produce a buffering effect in the higher alkaline region, such that their initial response to acidification is minimal. This results in large amounts of acid being necessary to initially lower the formulation pH when the starting pH of the lignin compositions is in the higher alkaline range, resulting in the creation of corresponding higher amounts of electrolytes.
Acidification of the lignin by the formulator and/or dyer not only produces undesirable salts in the dyestuff composition, but also is an added expense in preparation of the lignin additive for use.
Although ammonium lignosulfonate salts have been produced as a by-product of the ammonia-based sulfite pulping process, and ammonium naphthalene sulfonates have been produced in the oil refining field, ammonium salts of sulfomethylated lignins have not heretofore been produced effectively as a commercially viable by-product from the black liquor of the kraft pulping process. Such is due to the fact that when an aldehyde, such as formaldehyde (CH.sub.2 O), and ammonium sulfite [(NH.sub.4).sub.2 SO.sub.3 ] are added under alkaline conditions to sulfomethylate the lignin, multiple reactions occur. In addition to forming a desired soluble ammonium salt of the sulfomethylated lignin, ammonium hydroxide acts to combine with the formaldehyde and the lignin to produce a large amount of undesirable insoluble monomeric, dimeric and trimeric lignin products.
These reaction products are illustrated by the following empirical formulae: ##STR1##
Attempts to produce the soluble ammonium salt of sulfomethylated lignins, as described above, results in appreciable loss of the reaction materials in the form of insoluble polymeric lignin compounds.
Recent efforts have been directed in the energy field to the use of coal-water slurries as a fluid fuel to replace more costly oil and gas fuels commonly employed in power plant and boiler operations. Such coal-water slurries are made by grinding mine coal into a powder-like dust which is mixed with water and chemical additives to simulate fluid flow characteristics of conventional gas and oil fuels. The chemical additives in the coal-water slurries improve fluid flow characteristics and assist, as a dispersant, to maintain the coal particles in suspension in the slurries.
It has been proposed to use lignosulfonate salts as dispersants in coal-water slurries. U.S. Pat. No. 4,465,495 discloses the use of certain of such organosulfonate salts in coal-water slurries. However, the sodium salts of lignosulfonates have been found to produce scaling and corrosion of fuel burners and boilers. Ammonium salts of sulfomethylated lignin by-products of the kraft paper-making process have not been produced with sufficient economy to justify their use, due to the aforementioned creation of an undesired insoluble lignin portion.